Method of making manganese-nitrogen pre-alloy



United States Patent 2,989,429 METHOD OF MAKING MANGANESE-NITROGEN PRE-ALLOY Harry Tanczyn, Baltimore, Md., assignor to Armco Steel Corporation, a corporation of 01110 No Drawing. Filed Mar. 11, 1959, Ser. No. 798,610 1 Claim. (Cl.148--20.3)

My application is a continuation-in-part of my copend- Accordingly, one object of my invention is to provide a 7 method for the production of what I term a high-nitrogen content manganese-containing pre-alloy, of either the lowcarbon or medium-carbon type.

Yet another object is to produce a high grade manganese-containing pre-alloy of high nitrogen content in a manner which is at once simple, direct, reliable, and thoroughly practical and which effectively combines the factors of maximum efficiency and minimum cost.

Another object is to provide a method of economically and efiiciently producing a manganese-containing pre-alloy of high nitrogen content which pre-alloy readily lends itself to ease ofhandling and has wide range of application in the production of steels of high nitrogen content, particularly stainless steels for high temperature use.

Other objects in part will be obvious and in part pointed out hereinafter during the course of the following description.

Accordingly, my invention may be considered to reside in the several operational steps and in the relation of each of the same to one or more of the others, all as described herein, the scope of the application of which invention is indicated in the claims at the end of this disclosure.

(It will be helpful at this point to note that the highnitrogen manganese-containing pre-alloys serve as eifective sources of both nitrogen and manganese in the production of high manganese alloys of various types as particularly described and claimed in my copending application, Serial No. 205,602, filed January 11, 1951 and entitled Production of High-Nitrogen Manganese Alloy (now U. S. Patent 2,696,433 of December 7, 1954). As especially noted in that patent the pre-alloys are useful in the production of stainless steels intended for high temperature use, illustratively, for the manufacture of exhaust valves for internal combustion engines. Illustratively, such steels analyze about say 21% chromium, 4% nickel, 9% manganese, around 0.4% nitrogen, and the balance iron.

Advantageously, the high nitrogen content of my prealloys is combined with a high manganese content and possibly a high chromium content as well. Actually, I

find that these manganese-containing pre-alloys should contain manganese on the order of at least about 60% Through the use of the nitrogen-bearing high-manganese pro-alloys suflicient percentages of both nitrogen and manganese may be introduced into iron alloys to impart certain desired working properties to the metal, and to improve the grain structure thereof. The nitrided manganese-containing pre-alloys, however, do not occur in nature; they are prepared through proper processing technique, as more particularly described hereinafter.

An object of my invention, therefore, is the provision of a process for producing highqnitrogen bearing electrolytic manganese and medium-carbon and low-carbon term-manganese of high nitrogen contents, all in solid form, which is rapid and simple, certain and predictable in results, which has little efiect upon the carbon content of the pre-alloy, with close control of nitrogen content and which is practiced readily without substantial cost to give a clean, finished product ready for immediate use.

Referring now more particularly to the practice of my invention, the alloys which have high manganese content will hold substantial quantities of nitrogen. My investigations further disclose that under proper operating conditions, both electrolytic manganese and term-manganese of the low-carbon, or medium-carbon grades will take up a required high percentage of nitrogen. Generally stated, therefore, my invention contemplates washing lumps or particles of either electrolytic manganese or ferro-manganese, with nitrogen gas at elevated temperatures and for a time sufficient for required nitrogen pick-up into the metal.

My experiments disclose that best results are obtained by nitriting the solid material. I have found that the amount of nitrogen pick-up varies as a function of both the temperature to which the manganese-containing ingredient is subjected, and of the time of exposure.

Frequently, I find that, to avoid contaminating the metal to which the pre-alloy is added, the purity of the socalled pre-alloy itself must be exceptionally high. Illustratively, where high-nitrogen high-manganese pre-alloys are employed as an addition in stainless steel finishing operations, it frequently is necessary that the carbon content of this pre-alloy be maintained so low that it will not afiect to any material degree the carbon analysis of the steel. Moreover, I have observed that the carbon content of the pre-alloy itself appreciably affects the amount of nitrogen pick-up in the nitriding process.

Thus, in accordance with my invention, I find that the manganese-containing ingredients, either electrolytic manganese or medium-carbon or low-carbon ferromanganese, when held at a suitable high temperature in the immediate presence of nitrogen gas, absorb a considerable quantity of nitrogen. This penetrates the surface of the pro-alloy, reacting with the manganese of the prealloy to form stable manganese nitrides or perhaps solid solutions therewith. Further, I observe that the amount of nitrogen gas absorbed by the manganese-containing ingredient increases with increase in the duration of heat. Nitrogen penetration is observed to greater depths below the surface of the metal, with increase within certain limits of both the temperature and duration of heating.

In the practice of my invention I find that a certain criticality resides in the size of the particles of the solid material undergoing treatment, having important eifect on the percentage of nitrogen pick-up. More precisely, I have found that best results are achieved when the material undergoing nitrogenation is reduced to uniform particles of about the size of rice grains, this in the particular size had by crushing plates or flakes of electrolytic manganese. Particles of this size apparently combine most effectively the elements of high surface area per unit of volume, with sufiicient cross-sectional depth to permit penetration and assimilation of the nitrogen into the body of the manganese-containing ingredient. About equally suitable, however, I have found to be thin plates of electrolytic manganese, this on the order of one-sixteenth inch thickness. These pick up nitrogen about as well as do the grains.

Where desired, the electrolytic manganese or the ferro- Patented June 20, 1961 manganese may be. crushed to a powder, and then heated within the requisite temperature and time range later to be disclosed. In general, however, it is undesirable that the size of the grains of manganese-containing material be too small, this for the reason that the manganese prealloy ordinarily is added to the molten bath through a finishing slag and waste is observed when the lumps of pre-alloy are extremely small, due to their becoming suspended in the finishing slag. Accordingly, when the powdered manganese-containing ingredient is employed, I subsequently mix a suitable binder with the nitrided prealloy powder and briquet the same. The briquets are thereupon handled easily and without waste and have advantageous metallurgical utility because of increased density.

Now, in the practice of my process it is of course desirable, from the standpoint of economy, to operate at a lowest possible cost of materials. Thus, it is advantageous to employ comparatively cheap, commercial nitrogen gas, as contrasted with the relatively high cost of chemically pure gas or ammonia. Where the commercial nitrogen gas contains an objectionable amount of oxygen, or an objectionable amount of water vapor, I find that it is highly desirable to preliminarily de-oxygenate the gas. Illustratively, I pass the gas through crushedferro-silicon or crushed ferro-aluminum for this purpose. This prevents the oxidation of the manganese and provides an active surface to absorb nitrogen with predictable certainty.

In order to achieve the desired pick-up of nitrogen, and the realization of a product of high nitrogen content, I wash the electrolytic manganese, or the ferro-manganese, with nitrogen gas before bringing the same to desired temperature of treatment. For I find that where the manganese or ferro-manganese is brought to treating temperature in air, or without flushing out the air around and about the particles of product, there forms an oxide film in the product which precludes nitrogen pick-up in required manner and amount. And I also bring the nitrogen-treated product to room temperature, or somewhat above, before removing the same from the nitrogen atmosphere; I find that exposure to the atmosphere, while still hot, results in rapid oxidation with loss of metal by flaking off and a loss of desired nitrogen, as well.

Now as to temperatures of treatment, best results are achieved in heating the electrolytic manganese or the ferro-manganese in particles of required small size at a temperature ranging from about 1500 F. to the neighborhood of l900 F. in the presence of nitrogen gas. The time of treatment ranges generally from one to two hours or more. As a result it is observed that substantial quantities of nitrogen are picked up.

As specifically illustrative of the practice of my invention samples of electrolytic manganese, low-carbon ferromanganese and medium-carbon ferro-manganese were treated in accordance with my invention, the results of which are, respectively, reported in Tables I, II and III which follow. In the practice of all tests a suitable quantity of the pre-alloy in the form of plates for electrolytic manganese or lumps for the ferro-manganese, is placed in a nitriding box within a furnace, flushed with nitrogen gas, heated for required time at required temperature in the immediate presence of nitrogen gas, and then cooled almost to room temperature before discharge from the box. For this purpose it is entirely suitable to pass the nitrogen into one end of the nitriding box through a conduit and out the other end thereof through another conduit. The gas is supplied the box and contents at a pressure sufficient to supply nitrogen to replace that taken up by the metal or alloy and suificient to prevent entrance of atmospheric air through the exhaust end of the box. A pressure of some 2 or 3 psi. is sufiicient for this purpose. Thus, I ensure an adequate supply of nitrogen over and through the crushed pre-alloy.

TABLE I Nitridt'ng electrolytic manganese at various temperatures for /2 hour 6 Temperature of treatment: Nitrogen pick-up, percent 1500 F 2.23 1700 F 3.07 1900" F 5.00 2100 F 4 64 By way of comparison, I find that while the pick-up of nitrogen at 1900 F. is 5% for the electrolytic manganese plates, it is only about 2.8% where the manganese is melted and gaseous nitrogen bubbled through the bath, this for about minutes. And, from the foregoing it will be noted that the nitrogen pick-up is slightly less for a 2100 F. temperature than for a 1900 F. temperature, thus indicating that perhaps it is not desirable to operate at too high a temperature. Generally similar tests were made for a period of two hours with temperatures ranging from 1600 F. to 1700 F. and the nitrogen pick-up was found to be about 5.8%. With dry nitrogen gas for a period of approximately one-half hour, the nitrogen pick-up was observed to be 3.33%.

The nitrogen-bearing manganese achieved by treatment of electrolytic manganese in accordance with the teachings of my invention contains up to about 6% nitrogen, with the remainder substantially all manganese.

Where low-carbon ferro-manganese is employed, the following Table II illustrates the results achieved:

TABLE II Nitria'ing low-carbon ferro-manganese (85.29% manganese, 0.13% carbon, 0.89% silicon and remainder iron) in the form of particles of the general size of rzce grams at dzfierent temperatures and times at temperature Time of Nitrogen Temperature of Treatment, F. Treatment, Pick-up. hrs. percent TABLE III Nitriding of medium-carbon ferro-manganese at different temperatures and times at temperature Duration of Treatment, hrs.

Nitrogen Pick-up, percent;

Temperature of Treatment, F.

With high-carbon ferro-manganese (about 6% carbon, 70% manganese, and remainder iron) the maximum nitrogen content was obtained by treatment at 1800 F. for three hours, but this only amounted to 0.46%.

The composition of the nitrogen-bearing ferro-manganese had according to the practice of my invention then is: about 1 to 6% nitrogen, to 85% manganese, and remainder iron.

I have already called attention to the decreased tendency for nitrogen pick-up when operating about 1900 F. or higher. Apparently, when operating at this or higher temperatures, the particles of materialtend to sinter together, and it is likely that this sintering prevents good nitrogen pick-up. A further possible explanation for the criticality of the range lies in the fact that the manganese reacts differently under different temperature conditions. It is entirely possible that different solid solutions are obtained of the manganese and nitrogen; or perhaps different compounds are formed between these two elements within the range of temperatures below the melting point of the manganese or ferro-manganese. In any event, in the preferred practice of my invention, the temperature is maintained between about 1500 F. and about 1900 F. for a period of from about one to two hours. And it is thought that with the practice of my invention there is had Mn N, this containing 6% nitrogen by weight and remainder manganese. While other compounds of manganese and nitrogen very likely are formed they are unstable at the temperatures reached.

Certain advantages of my invention are had even within the broader temperature range of from 1200 F. up to about 2400 F. and the duration of treatment may range from one-quarter hour to about five hours or more. Good results are achieved when the gas used is commercial bottled nitrogen.

As a specific illustration of the practice of my invention in making up about a ton of the manganese-nitrogen pre-alloy, 'I charge 2000 pounds of electrolytic manganese plates or chips of manganese plates or clips of about inch thickness, all about 1 to 3 inches in breadth into a suitable container. The container is fashioned of 18-8 chromium-nickel stainless steel, 9 feet in length and 20" by 18" in width and depth, giving a total capacity of 22 /2 cubic feet. The electrolytic manganese is charged into one end of the container when standing on its other end, this to the extent of some 5 to 7 feet. The container then is laid onto a car and pushed into a heattreating furnace with the charging end protruding from the furnace. A cover is provided with gas inlet and outlet, the inlet connecting to a pipe extending to the op posite end of the container.

Nitrogen gas is then flowed through the container and the charge to flush out the air present. This continues as the temperature of the charge is gradually raised by the heat of the furnace. The gas itself is supplied by a battery of tanks of commencial nitrogen and is maintained at a pressure of 3 p.s.i. The flow varies as the process goes forward.

While for the specific example given the furnace is at a temperature of 1800 F. I find that the center of the charge actually reaches a somewhat higher temperature, about 2000 F. as a maximum. I attribute this to an exothermic reaction which seems to commence at a temperature of about 1400 F.

In the illustrative example of my process the complete cycle of treatment took about 12 hours time. About 3 or 4 hours were consumed in bringing the charge to the reaction temperature. During the initial part of this period the charge and container are completely freed of air so that oxidation and its inhibiting effect on nitrogen pick-up is effectively precluded. The time for the reaction, during which nitrogen is taken up at maximum rate, amounts to about 3 or 4 hours. To assure a maximum of nitrogen pick-up, however, the flow of nitrogen was continued another 4 hours after which the container was removed from the furnace, cooled to something just above room temperature and the contents discharged. The time for cooling the 2000 F. charge amounted to about 2 hours. The manganese-nitrogen alloy weighed 2120 pounds. It analyzed 5.7% nitrogen and remainder manganese. There was consumed about 2300 cubic feet of nitrogen gas (10 tanks), this amounting to 160 pounds. Of this it is noted that pounds was taken up by the manganese. The remaining 40 pounds was consumed in flushing out the charge in the first place and in maintaining sufficient flow to prevent air seeping back through the container outlet.

Highly advantageous results attend upon the practice of my new process in making high nitrogen manganese or high nitrogen ferro-manganese either of the mediumcarbon or low-carbon grades. No carbon is taken up in the process and with electrolytic manganese and lowcarbon ferro-manganese a low-carbon product is had. Thus the pre-alloy serves effectively as an additive to a steel bath where both the manganese and nitrogen contents are to be increased without material carbon increase. Equally successful is the use of the medium-carbon and low-carbon ferro-manganese of high nitrogen content, illustratively, valve steels where a substantial carbon content is desired.

According to my invention, there is provided a high manganese metal or alloy of high nitrogen content in simple, direct and reliable manner. The various objects hereinbefore noted are successfully achieved, together with many practical advantages. Moreover, by my new process I am enabled to produce nitrogen-containing iron-manganese alloys of desired nitrogen content, controlled within desired commercial limits.

Inasmuch as many embodiments are possible of my invention, and many modifications may be made of the embodiments illustrated, I desire that all matter herein be considered by way of illustration, and not by way of limitation.

I claim as my invention:

In the production of a nitrogen-manganese pro-alloy suitable for direct charge into a steel making furnace and containing about 2% to 6% nitrogen and remainder manganese, the art which includes washing a loose charge of electrolytic manganese plates with a flow of nitrogen gas in a closed container to purge the same of surrounding air; thereafter maintaining said charge in said container at a temperature of approximately 1200 F. to 2400 F. in the presence of nitrogen gas under pressure suflicient to effect nitrogen pick-up ranging from 2% to 6% by weight; and cooling said plates with picked-up nitrogen almost to room temperature While in the presence of said gas before discharging the same from the container.

References Cited in the file of this patent UNITED STATES PATENTS Epstein Nov. 17, 1942 Wanamaker et al. Nov. 11, 1958 

